Plastic materials are used in a wide variety of fields, making use of their impact resistance, lightweight and processability. Especially acrylic resin, polycarbonate resin and styrene-based resin which are transparent plastics are widely used as substitutes for glass. However, as these resins are unsatisfactory in terms of weatherability, it is known that when they are used outdoors for a long time, they decompose or deteriorate, thereby impairing their physical properties and appearances. Further, they have such defects that their surfaces are apt to be scratched due to their low abrasion resistance and affected by a solvent.
In recent years, there has been a trend toward the use of organic glass comprising a transparent plastic as a base in a windowpane, especially a car windowpane, making use of its lightweight and safety. Weatherability as high as that of glass is required for this organic glass. Since it is necessary to prevent the scratching of a windshield during the operation of a wiper and the scratching of a side window during the vertical movement of the window, excellent abrasion resistance is required.
To improve the weatherability and abrasion resistance of the organic glass, there have been made a large number of proposals for using an organosiloxane resin layer on the surface of a base. For example, patent document 1 and patent document 2 disclose a resin composition which contains a trihydroxysilane partial condensate and colloidal silica. Further, patent document 3 and patent document 4 disclose a resin composition which contains a condensate of an alkyltrialkoxysilane and a tetraalkoxysilane and colloidal silica.
Although cured films of these resin compositions have a certain measure of abrasion resistance, the cured films are pulled due to a difference in thermal expansion coefficient between the base and the cured films in a high-temperature environment and may crack. Therefore, they have a problem with durability.
To improve this, there have been made proposals for forming a thermosetting acrylic resin layer between the base and the organosiloxane resin layer. For example, patent document 5 proposes the use of an acryl-urethane resin layer which is obtained from an acrylic polyol and an isocyanate compound as the thermosetting acrylic resin layer. However, as the isocyanate compound has high reactivity, the storage stability of the obtained coating composition is low and the viscosity of the composition increases during use or storage, whereby it is apt to gel. Moreover, a side reaction readily occurs at the time of thermal curing.
Patent document 6 and patent document 7 propose the use of a vinyl-based copolymer containing an alkoxysilyl group as the thermosetting acrylic resin layer. However, as the alkoxysilyl group gradually reacts with water contained in the composition to be hydrolyzed and condensed, the composition becomes viscous and gels.
Patent document 8 discloses a resin composition which contains an alkylcyclohexylalkyl ester of (meth)acrylic acid as an essential component. The weatherability of this resin composition is not satisfactory and there is room for improvement.
Meanwhile, to improve weatherability, there is known a method in which an ultraviolet absorber is added to the thermosetting acrylic resin layer. For example, patent document 9 proposes that a benzotriazole-based ultraviolet absorber is added to a thermosetting acrylic resin to improve its weatherability. However, the optical decomposition rate of the ultraviolet absorber itself is high and there is limitation to the long-term suppression of the optical deterioration of the base. When a large amount of the ultraviolet absorber is added to improve this defect, adhesion between the base and the organosiloxane resin layer deteriorates.
A triazine-based ultraviolet absorber is known as an ultraviolet absorber having a low optical decomposition rate. However, when the triazine-based ultraviolet absorber is added to the thermosetting acrylic resin, the obtained cured film is apt to be whitened by its outdoor exposure and its appearance and transparency are easily impaired.    (Patent Document 1) JP-A 51-002736    (Patent Document 2) JP-A 55-094971    (Patent Document 3) JP-A 63-278979    (Patent Document 4) JP-A 01-306476    (Patent Document 5) JP-A 62-169832    (Patent Document 6) JP-A 59-109528    (Patent Document 7) JP-A 2004-131549    (Patent Document 8) JP-A 2002-206042    (Patent Document 9) JP-A 2000-318106